1. Field of the Invention
The invention relates in general to an electronic ballast of a fluorescent lamp (a daylight lamp), and more particularly, to a (vibration) stopper protection circuit of the electronic ballast of the fluorescent lamp.
2. Description of the Related Art
As illumination equipment has become a part of daily life for human beings, requirements such as suppressing flashing, increasing lifetime of the lamp tube, and enhancing the luminance efficiency are demanded more and more. To meet the above requirements, the electronic ballast has been developed.
The electronic ballast can be used to drive a signal fluorescent lamp tube or multiple fluorescent lamp tubes. When the electronic ballast is applied to drive multiple fluorescent lamp tubes, the operational characteristics of each lamp tube have to be similar. That is, the operation load has to be balanced. If some of the lamp tubes are partly out of order, such as through leaking, being inactive, being in the rectifying status at the end of lifetime or the load being short-circuited, a certain amount of error signal is generated due to unbalanced operation load.
While the electronic ballast is operated, the current of the power loop or the voltage of the lamp tube is sampled by the protection circuit of the electronic ballast, which uses the signal difference between the normal operation and error operation to provide protection. If the electronic ballast is used to drive multiple fluorescent lamp tubes and only some of the fluorescent lamp tubes are partly out of order, the sampling signal difference between the normal operation and the error operation is not significant. If different operation conditions such as low voltage or low temperature of the lamp tube are further considered, the sampling signal of the normal operation overlaps with the sampling signal of the partly out-of-order lamp tubes, and the critical value of protection action for the protection circuit cannot be appropriately determined.
The present invention provides a (vibration) stopper protection circuit of an electronic ballast for a fluorescent lamp. The electronic ballast has an inverter to convert a direct current source into a square-wave source and to provide the square-wave source to several lamp tubes. The stopper protection circuit includes several harmonic oscillating capacitors, a sampling transformer, a protection signal processing circuit and a (vibration) stopper circuit. The first terminals of the harmonic oscillating capacitors are electrically connected to one terminal of the lamp tubes. The sampling transformer has a first output winding and several detecting windings. The first terminals of the detecting windings are electrically connected to the other terminals of the lamp tubes. The second terminals of the detecting windings are electrically connected to the second terminals of the harmonic oscillating capacitors. The sampling transformer is used to generate a normal sampling signal and an error sampling signal, such that a significant difference between the normal sampling signal and the error sampling is obtained. The protection signal processing circuit is electrically connected to the output winding to receive the normal sampling signal and the error sampling signal, and to output an error signal while receiving the error sampling signal. The stopper circuit is electrically connected between the protection signal processing circuit and the inverter. While receiving the error signal, the stopper circuit outputs a stop signal to stop the operation of the inverter.
The stopper circuit comprises a load resistor, a diode and a controllable switch. The load resistor has a first terminal electrically connected to the positive electrode or the direct current source. The positive electrode of the diode is electrically connected to the inverter, while the negative electrode thereof is electrically connected to the second terminal of the load resistor. The controllable switch has a source terminal, a load terminal and a control terminal. The source terminal of the controllable switch is electrically connected to the second terminal of the load resistor and the negative electrode of the diode. The load terminal of the controllable switch is electrically connected to the negative electrode of the direct current source. The control terminal of the controllable switch is electrically connected to the protection signal processing circuit. The protection signal processing circuit includes a diode, a filter capacitor, a first delay resistor, a second delay resistor, a Zener diode and an interference-filtering capacitor. The positive electrode of the diode is electronically connected to one terminal of the output winding. The first terminal of the filter capacitor is electrically connected to the negative electrode of the diode, and the second terminal thereof is electrically connected to the other terminal of the output winding and the negative electrode of the direct current source. The first delay resistor has a first terminal electrically connected to the negative electrode of the diode and the first terminal of the filter capacitor. The first terminal of the second delay resistor is electrically connected to the second terminal of the first delay resistor, and the second terminal of the second delay resistor is electrically connected to the negative electrode of the direct current source. The first terminal of the delay capacitor is electrically connected to the second terminal of the first delay resistor and the first terminal of the second delay resistor. The second terminal of the delay capacitor is electrically connected to the first terminal of the negative electrode of the direct current source. The negative electrode of the Zener diode is electrically connected to the second terminal of the first delay resistor, the first terminal of the second delay resistor and the first terminal of the delay capacitor. The positive electrode of the Zener diode is electrically connected to the stopper circuit. The first terminal of the interference filtering capacitor is electrically connected to the positive electrode of the Zener diode and the stopper circuit, and the second terminal thereof is electrically connected to the negative electrode of the direct current source.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.